This invention relates to a cleanbox suitable for use in storing or transporting objects to be processed such as semiconductor wafers, photomasks and hard discs in an ultra-clean environment.
When transporting/storing substrates to be processed such as semiconductor wafers and photomasks used in production plants for semiconductor devices, minute amount of dust particles adhering to the objects such as semiconductor wafers and gaseous impurities existing in the atmosphere surrounding the objects lead to lower product yield. Such contamination problems become more critical as the density of circuit integration increases. For magnetic discs also, the advent of magnetic reluctance head has resulted in significantly accelerating high density recording so that product cleanliness is being demanded in terms of not only dust particles but gaseous impurities. From the viewpoint of reducing reaction of substrates, convenient means for maintaining low humidity are also important during storage of substrates.
The demand for such a clean environment in which to transport/store substrates has led to the development of cleanboxes equipped with high-efficiency particle air (HEPA) filter and ultra-low penetration air (ULPA) filter working in conjunction with a circulation fan. Also, other types of storage devices include replacing the air surrounding the wafers in the cleanbox with high purity nitrogen so as to maintain non-reacting environment and prevent the growth of native oxide film on the wafer surface.
However, although those cleanboxes based on HEPA and ULPA filters are able to remove particulate contaminants, they are not capable of removing minute amount of organic or inorganic gases. Furthermore, the efficiency of sweeping the environment is reduced and the effects of clean air in preventing contamination cannot be expect, when the circulation system allows a large volume of circulated air to flow into those regions that do not contribute to cleanliness of the environment, or if stagnation occurs within the cleanbox. As for the cleanboxes using a nitrogen-based atmosphere, such boxes are not able to remove impurities emanating from the wafers having some type of coating such as photoresist film, for example, and it also raises a concern related to the safety of nitrogen atmosphere to workers.
In view of the problems existing in the current technologies, the object of this invention is to provide a cleanbox to prevent not only contamination that can be introduced from external environment but also contamination that can be produced internally from the objects stored in the cleanbox and component parts of the cleanbox.
The invention relates to a cleanbox comprising: a casing container comprised of a container main body and a lid member for hermetically sealing a top opening section of the container main body; a dividing wall for forming circulation paths having an upstreaming path and a downstreaming paths within the casing container; a substrate holding section disposed in the upstreaming path for holding broad surfaces of substrates approximately parallel to the upstreaming path; an air filter and a gas removal filter disposed upstream in relation to the substrate holding section in the upstreaming path; and a motor-driven fan housed in the casing container for producing air streams circulating in the circulating paths.
Accordingly, circulation paths are created within the container, and the flowing streams are purified first by using air filter and a gas removal filter to remove contaminants physically and chemically and then the air streams are directed to the substrate holding section, so that, even if there are particulate and gaseous sources of contaminants adhering to the inner walls of the container or to the wafers themselves, contamination of the wafers stored in the substrate holding section can be prevented. Because the air filter and the gas removal filter in the upstreaming path are placed upstream in relation to the substrate holding section, objects to be cleaned can be readily loaded or unloaded from the top section of the container without the fear of causing contamination to the objects. Also, because the container is quite susceptible to contamination when the lid is opened or closed, the container lid is placed downstream in relation to the substrate holding section, so that any contaminants that may have been brought into the container can be removed long before they can reach the objects.
The filtering membrane of the gas removal filter may be comprised singly of ion exchange fibers or activated charcoal fibers, or by combining these fibers or by activated charcoal particulate embedded in a urethane foam carrier, or by using integrated filtering unit made by weaving these fibers together. The filtering system efficiently removes and adsorbs ions such as ammonia present in the air and ionic substances contained in a mist such as hydrofluoric acid and hydrochloric acid by using ion exchange fibers and activated charcoal fibers produced by activation reaction of carbons produced from cellulose, acrylic and lignin group fibers. Ion exchange fibers may be produced by radiation-induced graft polymerization.
According to another feature of the invention, the upstreaming path is formed approximately centrally in the casing container, and merges with the downstreaming paths formed between the dividing walls and the container main body, by being redirected at a top section and/or a bottom section having an arc surface or a spherical surface. By shaping the top section and/or bottom section into a dome shape having an arc radius or spherical radius, the overall flow is made smooth with no stagnating points, and therefore, the energy required to form circulating paths is reduced so that one battery is sufficient to operate the cleanbox for a prolonged period of time. It is preferable that the radius of curvature be not less than 1.2R when the object wafer radius is R.
According to yet another feature, the lid section at the inner surface and the container main body at the bottom surface are provided with guiding protrusions for separating the streaming path. Normally, the profile of such a protrusion is triangular.
According to yet another feature, the circulation paths are provided with a reverse flow preventing filter in a downstream location to the substrate holding section so as to prevent back streaming of contaminants from the motor-driven fan. Accordingly, even if the fan is stopped, there is no danger of contaminating the wafers due to contaminants in the fan unit.
It is preferable that the motor for the motor-driven fan is a dc brushless motor, because there are no moving parts to generate contaminants. Furthermore, the motor may be canned in a stainless steel casing of 0.2 mm thickness, so that the motor is dustless and gas tight, thereby preventing contamination by gaseous substances emitted from the coils and other parts of the fan.
According to yet another feature, the substrate holding section is a carrier receiving section for freely detachably seating a carrier having openings at the top and the bottom sections, at least, for holding a plurality of substrates. This arrangement enables the wafers to be transported in carrier-based quantities.
According to yet another feature, the substrate holding section is provided with guide grooves for directly supporting the plurality of substrates. Thus, the objects are stored without using the carrier so that the cleanbox can be made more compact and light weight.
According to yet another feature, the substrate holding section houses a plurality of substrates, and the substrate holding section is provided with a diffuser plate for dispersing circulating air in the spaces between the substrates. Accordingly, cleaning of the wafers can be carried out efficiently by producing uniform flow of air in the spaces between the wafers.
According to yet another feature, a cleanbox recited in claim 1, in which the motor-driven fan is operated by a control section. Accordingly, sequences for different mode of operation may be programmed into a microchip, so that intermittent operation may be carried out, or the fan speed may be varied to prolong the service life of the battery to improve the efficiency and economy of operating the cleanbox.
According to yet another feature, the container main body is structurally integrated with constituting component parts in the container main body without using fasteners. Accordingly, contamination generated from the fasteners and other means of joining are prevented while preserving the ease of handling the components of the cleanbox for maintenance and replacement.
According to yet another feature, the container main body is maintained at a low humidity by using a dehumidifying agent made in a sheet form or stored in a bag. Accordingly, low humidity levels in the cleanbox can be achieved quickly by circulating the air inside the cleanbox by means of the fan.
According to yet another feature, the gas removal filter has a filtering membrane made of a non-woven ion exchange fabric or a woven ion exchange fabric produced by radiation-induced graft polymerization.
According to yet another feature, the gas removal filter has a filtering membrane comprising activated charcoal produced by imbedding activated charcoal particulate in activated charcoal fibers or urethane foam.
According to yet another feature, the gas removal filter has a filtering membrane comprising a combination of a gas removal filter having a filtering membrane made of a non-woven ion exchange fabric or a woven ion exchange fabric produced by radiation-induced graft polymerization and a filtering membrane comprising an activated charcoal produced by imbedding activated charcoal particulate in activated charcoal fibers or urethane foam.